[Polymer Fuel Cell]
In recent years, new energy storage or generation devices have been strongly demanded from the viewpoint of environmental problems. Fuel cells are receiving great attention as one of those devices and are the most prospective generation devices also in view of the characteristics of low-polluting emission and high-energy efficiency. Fuel cells are devices to electrochemically oxidize a fuel such as hydrogen, methanol, etc. using oxygen or air thereby to convert the chemical energy of the fuel into electrical energy and to take the energy out.
Such a fuel cell is classified by the type of electrolyte used in the cells into phosphoric acid type, molten carbonate type, solid oxide type and polymer electrolyte type. Phosphoric acid type fuel cells are already in practical use for electric power. However, phosphoric acid fuel cells are required to operate at high temperature (approximately 200° C.), and for this reason, has involved drawbacks that start-up time is long, downsizing the system is difficulty and a large current can not be taken out due to poor ion conductivity of phosphoric acid.
By contrast, polymer electrolyte type fuel cells operate at temperature of approximately 80 to 100° C. at maximum. Also, since an internal resistance in fuel cells can be lowered by reducing the thickness of electrolyte membrane used, the polymer electrolyte type fuel cells can be operated at a high current, which makes downsizing possible. From these advantages, polymer electrolyte type fuel cells have been actively pursue.
Polymer electrolyte type fuel cells include a type using as fuel pure hydrogen fed from a cylinder, piping, etc., a type using hydrogen generated from gasoline or methanol through a reformer, and the like. Furthermore, direct methanol fuel cells (DMFC) which generate electricity directly from an aqueous methanol solution as fuel have been developed. This DMFC need not a reformer for generating hydrogen, enables to make up a simple and compact system, and is expected especially as power sources devices. A polymer electrolyte type fuel cell consists of a polymer electrolyte membrane having ion conductivity arranged in contact with a cathode and anode on both sides of the membrane. Fuel hydrogen or methanol is oxidized electrochemically at the anode, producing protons and electrons. The protons migrate through the polymer electrolyte membrane into the cathode where oxygen is supplied. On the other hand, the electrons produced at the anode pass through the load connected to the cell, flow into the cathode, at which the protons react with the electrons to produce water.
The electrodes such as cathode and anode used in polymer electrolyte type fuel cells are composed of conductive materials having electric conductivity, electrode materials such as catalysts for promoting the oxidation of hydrogen and the reduction of oxygen, and binders for fixing these materials.
[Polymer Electrolyte Membrane Using Fluorinated Polymer Having a Protonic Acid Group]
In a polymer electrolyte membrane used for this polymer electrolyte type fuel cell, high ion conductivity is required for protons that take part in an electrode reaction of the fuel cell. Fluorinated polymers having a protonic acid group are employed as materials for such ion conductive polymer electrolyte membranes.
However, this fluorinated polymer electrolyte membrane having a protonic acid group is known to cause a drastic decrease in ion conductivity under higher temperature and lower humidity. In the polymer electrolyte membrane using fluorinated polymer having a protonic acid group, the hydrophobic parts on the main chain and the hydrophilic parts on the side chain form a micro-domain structure, and the hydrophilic parts are interconnected in a highly hydrated state to form ion conduction pathways due to the formation of water clusters. Thus, there has been a problem that ion conductivity decreases rapidly, since the conduction pathways is blockaded with evaporation of water under high temperature and low humidity. Moreover, the conduction pathways by water clusters permit methanol to permeate therethrough. For this reason, DMFC using the polymer electrolyte membrane using fluorinated polymer having a protonic acid group has involved problems with a reduced voltage and poor power generation efficiency due to the methanol crossover. Besides, the fluorinated polymer having a protonic acid group has involved additional problems that this polymer has as poor adhesion property as other fluorinated resins that makes it difficult to adhere to electrodes or membranes, is very expensive, and generates hydrofluoric acid gas during incineration in discarding.
[Polymer Electrolyte Membrane Using Hydrocarbon-Based Polymer Having a Protonic Acid Group]
On the other hand, non-fluorinated electrolyte membranes using hydrocarbon-based polymers having a protonic acid group have also been developed. In a hydrocarbon-based polymer electrolyte membrane, the formation of water clusters or ionic conduction pathways is not observed, and in its structure the entire electrolyte membrane is uniformly hydrated. Thus, it is known that in the hydrocarbon-based polymer electrolyte membrane, the polymer is highly interactive with water molecules, and the reduction in ion conductivity under high temperature and low humidity is diminished.
However, hydrocarbon-based polymers having a protonic acid group whose main chain consists of aliphatic chains have involved a problem that since these polymers provide poor heat resistance and chemical durability, the cell performance is deteriorated with passage of time when the polymers are used as polymer electrolytes for polymer electrolyte type fuel cells. For example, sulfonated polystyrene has involved a problem that since a glass transition temperature of the polymer is about 125° C., the membrane is softened and deformed by heat developed during power generation. There has been another problem that a tertiary carbon in the main chain is liable to be attacked by radicals and hydrogen at the α-position is easily released in the cell.
Thus, many aromatic hydrocarbon-based polymers having a protonic acid group have been developed, which do not have aliphatic chains in the main chains. (Macromol. Chem. Phys., 199, 1421-1426 (1998), Polymer, 40, 795-799 (1999), Polymer, 42, 3293-3296 (2001), etc.). Among them, membranes formed of sulfonated polyetheretherketone are reported to provide excellent heat resistance and chemical durability to be tolerant of long hour-operation as a polymer electrolyte (Itaru Honma, The 3rd. Separations Sciences & Technology Workshop Session, Lecture Brief, “Basis & Application of Polymer Membrane Fuel Cell,” p. 17 (1999)).
In order to improve ion conductivity of these aromatic hydrocarbon polymers having a protonic acid group, it is necessary to increase the content of protonic acid groups to be introduced, i.e., to reduce the ion-exchange equivalent weight. However, it is known that when the content of protonic acid group to be introduced increases, hydrophilicity increases, water absorption increases or the polymers become water-soluble (Japanese Patent Laid-Open Application Hei 10-45813, etc.). Since fuel cells produce water as by-product by the reaction of fuel and oxygen, water-soluble resin cannot be used as a polymer electrolyte membrane for fuel cells. Even if polymers do not become water-soluble, high water absorption causes problems such as swelling or reduced strength of membranes, permeation of methanol from the anode to the cathode mediated by the absorbed water, etc. In order to obtain polymer electrolytes having a high ion conductivity, it was thus required to reduce water solubility and water absorption of membranes.
International publication WO 00/066254, Japanese Laid-Open Patent Application SHO 63-305904, etc. disclose a method of reducing the water solubility and water absorption by blending a sulfonated resin having water solubility or water absorption with a basic polymer having neither water solubility nor water absorption to form a salt. However, since this method requires for incorporating basic polymer having neither water solubility nor water absorption in large quantities, the resulting polymer has a problem that the proportion of resin having protonic acid group resin is low and the ion conductivity is low. Moreover, there is further problem that since the salt formation is an equilibrium reaction and thus, for example, dissociation and recombination are repeated by a protonic acid that migrates through the membrane during power generation, the sulfonated resin is gradually dissolved out.
[Crosslinked Resin Membrane Having a Protonic Acid Group]
On the other hand, across linked structure through a covalent bond is attracting interest as a method for reducing water solubility without introducing water-insoluble components, which can also diminished the resin to dissolve out.
For example, Japanese Laid-Open Patent Application SHO 52-91788 reports a crosslinked resin membrane having a protonic acid group obtained by sulfonating the polyphenylene oxide membrane crosslinked through a Friedel-Crafts reaction using fuming sulfuric acid. However, sulfonation of the crosslinked resin membrane involves a problem that crosslinking density or sulfonation degree of the membrane surface is different from that of the inside or back surface of membrane, and membrane thickening is difficult. Furthermore, in this method, when producing a fuel cell electrode which is dispersed with a conductive material having electric conductivity or with a catalyst for promoting oxidation of hydrogen and reduction of oxygen, the conductive material or catalyst should first be dispersed in resin to form membrane and then sulfonation must follow, by which the conductive material or catalyst was unavoidably degenerated or deteriorated.
Though a method for crosslinking sulfonated resins during or after membrane formation is disclosed in Japanese Laid-Open Patent Application HEI 2-248434 and 4-130140, etc., this crosslinking mechanism that uses a Friedel-Crafts reaction of an aromatic ring and a chloroalkyl group is accompanied by eliminated hydrochloric acid as by-product. Thus, the method involves a problem that the crosslinking density is different between the membrane surface and the inside or back surface of membrane with a different efficiency of removing the by-product, membrane thickening is difficult, voids are formed on the membrane, and manufacturing equipments corrode due to the eliminated hydrochloric acid.
Another crosslinking mechanism is reported in, e.g., Japanese translation of PCT Publication 2000-501223, U.S. Pat. No. 6,221,923, etc., in which sulfonic acids in a sulfonated polyetheretherketone membrane are condensed by desulfuric acid each other to form sulfonate bonds. In Japanese Patent Laid-Open Application SHO 52-99982, a method which comprises thermally decomposing a part of chlorosulfonic acid of a membrane formed of chlorosulfonated polydiphenylmethane to form the crosslinkage through sulfonate bond and then hydrolyzing the remaining chlorosulfonic acid to sulfonic acid is reported. In International publication WO 99/61141, a method for crosslinking through sulfonamide bonds using chlorosulfonated polyetheretherketone and diamine is reported. However, these methods are also based on the crosslinking mechanism involving elimination of sulfuric acid, hydrochloric acid, chlorine, etc., and have a problem that the crosslinking density of the membrane surface is different from that of the inside or back surface of membrane, membrane thickening is difficult, voids are formed on the membrane, and manufacturing equipments corrode due to the elimination of acidic gas. In addition, the membranes prepared through the crosslinking mechanism using these protonic acid groups involves a further problem that increased crosslinking density invites reduced protonic acid groups (increased ion-exchange equivalent weight) to cause reduced ion conductivity. An additional problem was that the sulfonamide bonds were liable to hydrolysis.
For a crosslinking mechanism which is not accompanied by eliminated sulfuric acid, hydrochloric acid, etc. off, Japanese Laid-Open Patent Application HEI 6-93114 reports a mechanism that the chlorosulfonic acid group in chlorosulfonated polyetherketone reacts with an allyl amine to form an allyl group bonded through a sulfonamide group and the resulting polyetherketone is crosslinked through an addition reaction after forming the membrane. According to this mechanism, the degree of sulfonation or crosslinking density in a thickness direction can be made uniform. However, this method requires to reduce protonic acid groups (increase the ion-exchange equivalent weight) as well, in order to increase the crosslinking density. Moreover, the method also involves that the sulfonamide bond for binding crosslinkable groups is liable to hydrolysis.
Though a variety of crosslinking mechanisms have been proposed to crosslinked resin not having a protonic acid group, there are hardly found that crosslinking would take place even under conditions that protonic acid groups are present in large quantities, no reaction with protonic acid groups would occur, groups or chains produced by crosslinking would be stable in fuel cells, etc. For example, the curing system for epoxy resin or bismaleimide resin encounters problems that the epoxy or amine reacts with protonic acid groups to reduce the protonic acid groups, the chains produced are liable to hydrolysis or electric degradation, etc. Thus, the curing system cannot be used as a crosslinking mechanism for resins having protonic acid groups or as a polymer electrolyte membrane for fuel cells. For this reason, notwithstanding that various crosslinking mechanisms have been applied to conventional crosslinked resins, only a very few types of the crosslinking mechanisms described above are used for resins having protonic acid groups.
In view of the foregoing, crosslinkable aromatic resin having a protonic acid group as a polymer electrolyte membrane material for fuel cells, which has a crosslinking mechanism that can be crosslinked during or after membrane formation, that is not derived from the protonic acid group, and that does not forming any elimination component, has been required.
[Binder Having a Protonic Acid Group for Fuel Cell]
In the polymer electrolyte type fuel cells, very few reports are found on binders used to fix electrode materials or adhere the electrodes to a membrane, and only a fluorinated polymer having a protonic acid group is employed. However, while this fluorinated polymer having a protonic acid group adheres to a polymer electrolyte membrane formed of the fluorinated polymer having a protonic acid group, the polymer provides poor adhesion property to an aromatic hydrocarbon-based polymer electrolyte membrane having a protonic acid group.
Also, since binders are used for blending with electrode materials or adhering electrode materials and membranes, solvent solubility or melt flowability is required for binders. Therefore resins that have already been crosslinked to become insoluble or non-fusible can not be used as binders.
Therefore, ion conductive binders for fuel cells, which can be crosslinked during or after adhesion, and after the crosslinking, exhibit excellent ion conductivity, heat resistance, water resistance and adhesion property, have also been demanded.